fix(app): warm up model at startup to avoid first-request connection errors

[ousamabenyounes]

Load the VoxCPM model during server startup in run_demo() instead of lazily on the first request. The multi-GB from_pretrained ran inside the first click, blocking that request for minutes and surfacing any load/download failure as an opaque connection error (issue #367). Warming up before launch fails fast and keeps the first generation responsive.

Test verification (RED → GREEN)

With the fix reverted, the new test fails (RED):

platform linux -- Python 3.14.4, pytest-9.0.2, pluggy-1.6.0 -- /usr/bin/python3
cachedir: .pytest_cache
rootdir: /home/ousama/contribute-work/OpenBMB__VoxCPM
configfile: pyproject.toml
plugins: typeguard-4.4.4
collecting ... collected 1 item

tests/test_app_warmup.py::test_model_is_warmed_up_before_launch call order: ['launch']
FAILED

=================================== FAILURES ===================================
____________________ test_model_is_warmed_up_before_launch _____________________

    def test_model_is_warmed_up_before_launch():
        app = _load_app_module()
    
        calls: list[str] = []
    
        class _FakeInterface:
            def queue(self, *args, **kwargs):
                return self
    
            def launch(self, *args, **kwargs):
                calls.append("launch")
    
        with mock.patch.object(app, "create_demo_interface", return_value=_FakeInterface()), \
             mock.patch.object(app.VoxCPMDemo, "get_or_load_voxcpm",
                               autospec=True,
                               side_effect=lambda self: calls.append("warmup")):
            app.run_demo(device="cpu")
    
        print(f"call order: {calls}")
>       assert "warmup" in calls, "model was never warmed up (loads lazily on first request)"
E       AssertionError: model was never warmed up (loads lazily on first request)
E       assert 'warmup' in ['launch']

tests/test_app_warmup.py:82: AssertionError
=========================== short test summary info ============================
FAILED tests/test_app_warmup.py::test_model_is_warmed_up_before_launch - Asse...
============================== 1 failed in 0.07s ===============================

With the fix applied, the test passes (GREEN):

GREEN attempt 1/2 (exit 0)
============================= test session starts ==============================
platform linux -- Python 3.14.4, pytest-9.0.2, pluggy-1.6.0 -- /usr/bin/python3
cachedir: .pytest_cache
rootdir: /home/ousama/contribute-work/OpenBMB__VoxCPM
configfile: pyproject.toml
plugins: typeguard-4.4.4
collecting ... collected 1 item

tests/test_app_warmup.py::test_model_is_warmed_up_before_launch call order: ['warmup', 'launch']
PASSED

============================== 1 passed in 0.06s ===============================

Full local suite

Command: python3 -m pytest tests/ -v

============================= test session starts ==============================
platform linux -- Python 3.14.4, pytest-9.0.2, pluggy-1.6.0 -- /usr/bin/python3
cachedir: .pytest_cache
rootdir: /home/ousama/contribute-work/OpenBMB__VoxCPM
configfile: pyproject.toml
plugins: typeguard-4.4.4
collecting ... collected 46 items / 2 errors

==================================== ERRORS ====================================
____________ ERROR collecting tests/test_lora_checkpoint_loading.py ____________
ImportError while importing test module '/home/ousama/contribute-work/OpenBMB__VoxCPM/tests/test_lora_checkpoint_loading.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/lib/python3.14/importlib/__init__.py:88: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
tests/test_lora_checkpoint_loading.py:9: in <module>
    import torch
E   ModuleNotFoundError: No module named 'torch'
__________________ ERROR collecting tests/test_model_utils.py __________________
ImportError while importing test module '/home/ousama/contribute-work/OpenBMB__VoxCPM/tests/test_model_utils.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/lib/python3.14/importlib/__init__.py:88: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
tests/test_model_utils.py:20: in <module>
    spec.loader.exec_module(utils)
src/voxcpm/model/utils.py:3: in <module>
    import torch
E   ModuleNotFoundError: No module named 'torch'
=========================== short test summary info ============================
ERROR tests/test_lora_checkpoint_loading.py
ERROR tests/test_model_utils.py
!!!!!!!!!!!!!!!!!!! Interrupted: 2 errors during collection !!!!!!!!!!!!!!!!!!!!
============================== 2 errors in 0.17s ===============================

Fix #367